This invention relates to an idling rpm control method for internal combustion engines, and more particularly to a method of this kind which is intended to improve the fuel consumption characteristic of the engine while the engine is operating in an idling condition.
An internal combustion engine for automotive vehicles is generally disposed to rotatively drive an alternating-current generator which supplies electric power to electrical load-creating equipments such as a heater and air conditioner and lighting systems provided in the automotive vehicle. With this arrangement, if one or more of such electrical load-creating equipments are turned on, the field current of the generator is increased by the action of a regulator provided in the generator so that the generator operates to increase its own output power. However, while the engine is operating in an idling condition, the phenomenon can occur that the amount of electric power required by the electrical load-creating equipment exceeds the maximum output power of the generator that is determined by the output torque of the engine and the rotational speed of the generator. On such an occasion, a battery provided in the engine supplies electric power to the electrical load-creating equipment to compensate for a resultant shortage of electric power. Consequently, the battery can be overdischarged to have its effective life reduced, and the resultant reduction in the voltage of the battery can cause degraded startability of the engine, and other inconveniences.
To eliminate the above inconveniences, an idling rpm feedback control method has been proposed (e.g. by Japanese Patent Provisional Publication (Kokai) No. 54-155317) which comprises setting the desired idling rpm at such a value that the generator can produce electric power in an amount enough to compensate for the electrical loads applied thereon by the electrical load-creating equipment, and supplying an air/fuel mixture to the engine in a quantity corresponding to the difference between the desired idling rpm and the actual engine rpm, to thus control the rotational speed of the engine to the desired idling rpm in a feedback manner. However, according to this proposed method, the desired idling rpm has to be set at a certain high value such that the generator can produce sufficient electric power even if all the electrical load-creating equipment is operating at the same time while the engine is in an idling condition. Consequently, the rotational speed of the engine is maintained at the above high desired idling rpm even when no electrical load-creating equipment is operating, resulting in increased fuel consumption of the engine.